1. Field of the Invention
The present invention relates to a matrix type liquid crystal display device using an anti-ferroelectric liquid crystal medium.
2. Description of Related Art
Conventionally, a matrix type liquid crystal display device using an anti-ferroelectric liquid crystal medium is designed, for example, as shown in FIGS. 14A-14D, to display an image by driving scanning electrodes and signal electrodes while scanning the scanning electrodes according to the sequence of scanning lines so that a blanking period during which picture elements are dark displayed, a selecting period during which only picture elements necessary for bright display are inputted, and a holding period during which a pre-existing display condition is maintained, are repeated on a time-series basis.
In general, a picture signal such as a video signal is inputted to a liquid crystal display device in the form of a serial picture data so that a picture screen can be scanned both horizontally and vertically according to the drive mode of the display.
Thus, the selecting cycle of the scanning electrode has to be equalized to the vertical cycle of the picture signal to be inputted, unless the picture signal undergoes special data conversion processing. That is, the variation of the transmission cycle of image data for one picture frame directly corresponds to the picture producing cycle of the picture frame.
For example, in the case of the NTSC system, when a horizontal scanning period is 1 H, the frame period is 525 H. Also, in the case of the CRT, the field consists of odd-numbered fields and even-numbered fields in order to enable interlace scanning.
However, in the case of the liquid crystal display device, in general, interlace scanning is not carried out. Instead, the odd-numbered fields and the even-numbered field are superimposed on each other and displayed on a single display screen. Thus, the selecting cycle of the scanning electrode for displaying the odd-numbered fields differ by 1 H at a minimum from that for displaying the even-numbered fields.
In this case, the anti-ferroelectric liquid crystal medium needs to be driven with alternating current; however, if the voltage polarity is inverted from positive to negative or vice-versa for each selecting period, the difference in the selecting period corresponds to the difference in duration of the holding period. on the other hand, the voltage for maintaining the display condition is applied during the holding period. Thus, a DC voltage is applied to the anti-ferroelectric liquid crystal medium according to the above-described difference in duration of the holding period. This leads to phenomena such as burning or flickering of the display screen. These phenomena will be discussed in detail in the following with reference to FIGS. 14A-14D. Since the blanking period for each picture frame is constantly 4 H, the holding period of the picture frame corresponding to the odd-numbered field becomes 257 H, while the holding period of the picture frame corresponding to the even-numbered field is 258 H. When the polarity of the drive voltage is inverted for each selecting period, the difference in holding period by 1 H is applied, as a DC voltage, to the anti-ferroelectric liquid crystal medium, and this causes the above-described phenomena.